Hello friends 
Sorry, I don't speak French so I'm having to guess at the discussion. My second dyno was a very important one as it showed that a simple change of spark plugs could net a huge amount of HP gain. Many of my friends had not changed their plugs due to manufacturers ratings of the plugs. My group is pretty confident that changing the plugs around 13,000 miles should be a normal maintenance item. Coils and plug wires have also been the topic of discussion on rx8club.com. (See thread links below)
I noticed you have a download from my site in reference to the dyno day we had with our local club. The link is a direct download and without seeing the page it comes from it may be out of context. I hope you will check out the page of our event and see if it helps in your discussion.
Thanks for viewing my vids.
SilverEIGHT Dyno page: http://silvereightstudio.com/dyno/index.htm
Relevant threads on topic from rx8club.com:
http://www.rx8club.com/showthread.php?t=81408
http://www.rx8club.com/showthread.php?t=77031
Thanks again
Don
EDIT:
To clarify the thinking of a spark plug change at 13,000 mi (21,000 km), my thinking is that to maintain optimum performance, a change prior to the degradation of the plugs would be wise. If my plugs were fried at 18,000 mi with a loss of 28 - 30 hp then replacing them earlier would make sense to me, in my humble opinion.